The present invention is directed toward a sweep signal generation system for producing a signal having a swept frequency. The present invention is applicable to the production of an output sweep signal of frequency in the very high radio frequency range, of the order of 4 GHz, for example.
In the following, it should be noted that the term "output sweep signal" refers to a signal whose frequency is periodically swept through a predetermined range. The term "sweep repetition signal" as used herein refers to a signal which varies in a periodic manner, as by a periodic variation in amplitude, and is applied to a sweep signal generating circuit for controlling that circuit to produce a sweep signal whose varies in accordance with the variation of the sweep repetition signal. Systems for producing sweep signals at high radio frequencies which are available at present can be broadly classified into two types. The first of these types is characterized by a sweep repetition signal driving a voltage controlled oscillator, such as a YIG tuned oscillator, in a stepless manner. The second type is typified by the use of a frequency synthesizer circuit to produce various required frequencies in a stepwise manner. The first type of system has the advantage of simplicity, but has the disadvantage of a relatively low degree of frequency stability. The second type of system provides a high degree of frequency stability, but cannot provide a continuous, stepless variation of the frequency of the output sweep signal. In order to increase the resolution of such a system, i.e. to reduce the spacing between the stepwise variations of the sweep signal frequency, it is necessary to increase the time required to perform a sweep. This makes observation of the sweep signal on a display such as a cathode ray tube difficult. The second type also has the disadvantage of complex and therefore costly circuitry.
There is therefore a requirement for a sweep signal generation system which combines relatively simple circuitry with a high degree of frequency stability, and which can provide a stepless, continuous variation in frequency of the sweep signal. These objectives are met by the present invention.